1. Field of the Invention
The present invention relates to a method and apparatus for encapsulating semiconductor devices in an automatic molding system which complies with the high volume output requirement for semiconductor manufacturing. In particular, the present invention pertains to a direct drive electro-mechanical press for encapsulating semiconductor devices in a clean room environment which is ideal for in-line semiconductor manufacturing systems. By in-line manufacturing systems, the present invention refers to systems where all the machines necessary for producing the end product are laid out in line, thus minimizing the transportation and handling of the product which is in process between machines.
2. Description of the Related Art
Traditionally encapsulation of semiconductor devices is accomplished by using a transfer hold tool mounted onto a press. The press serves two functions: (1) the press provides the clamping tonnage to clamp the molds together in order to prevent bleeding and flashing of the resin during the encapsulation process, and (2) the press provides the transfer action to transfer the molding resin compound into the mold cavities. The presses used for molding semiconductor devices in the encapsulation process are hydraulic based. Examples of hydraulic presses include U.S. Pat. No. 4,599,062 and Japan Patent No. J5 9204-244-A.
The main disadvantages of a hydraulic transfer press are the contaminations from hydraulic leakages and oil fumes generated from the hydraulic pump system. As a result, the molding operation using a hydraulic press has to be isolated from the front-end of the encapsulation process which involves die attach, wire bonding and etc. because those operations are performed in a clean room environment.
As semiconductor fabrication operations are moving towards in-line systems, it is essential to integrate various processes including the molding operation in a clean room environment. An electro-mechanical press using a toggle mechanism has been developed recently to overcome the problems encountered during the molding process using a hydraulic press.
While the electro-mechanical press for encapsulating semiconductor devices overcomes the problems of the prior art systems featuring a hydraulic press, it has two major disadvantages: high wear at the joints, and disproportionate drive control.
The toggle mechanism used in an electro-mechanical press consists of a plurality of linkages and joints. In semiconductor encapsulation process under an in-line system, the linkages and joints experience high rate of wear and tear. As such, frequent maintenance and down time are incurred to prevent the failure of the total system.
The toggle mechanism in an electro-mechanical press relies on the principles of mechanical advantage. It follows that the output from the servo motor is used as an input to the plurality of linkages and joints. The disadvantage of the toggle mechanism lies in the disproportionate amplification of the input to the linkages and joints. In other words, the amplified output of the servo motor from the toggle mechanism is not constant. The magnitude of amplification depends on the angles between the output links and the input links. In a typical press for encapsulating semiconductor devices, there are at least four links called the tie bars. The amplification factors increase exponentially during the last phase of clamping. As a result, the disproportionate amplification of the servo motor output can either over-clamp or break the tie bars.
The present invention comprises a direct drive electro-mechanical press for encapsulating semiconductor devices. The press has at least two axes-a clamping axis and a transfer axis. The clamping axis comprises a planetary roller screw coupled to a tie bar platen assembly on which at least one top mold is mounted. A two-speed gearbox is further coupled to the clamping axis for switching between a high speed mode and a high torque mode along the clamping axis. The two-speed gearbox comprises a high speed clutch coupled to a speed reducer and a high torque clutch respectively. The transfer axis is slidably located above the clamping axis; the transfer axis comprises another planetary roller screw coupled to a bottom platen on which at least one bottom mold is mounted. The transfer axis transfers the molding resin from the pot into the cavities formed when the top and bottom molds are clamped together. The use of two-speed gearbox and planetary roller screw along the clamping axis and the use of planetary roller screw along the transfer axis not only reduce the number of joints or links required under prior art systems, but also permit the press to deliver superior velocities and clamping tonnage.
A closed-loop control is provided to the clamping and transfer axes respectively for minimising the disproportionate drive from the electro-mechanical press. In addition to the controllers and drivers, the closed-loop control provide direct feedback on the clamping tonnage on the tie bars and transfer pressure in the mold blocks. While better system control is achieved, problems associated with disproportionate drive from the servo motors such as breakdown due to over-clamping are minimized.